event parsing, tests, real -> fixed

- changed the type real to fixed
- expanded the types handled by the _canonical_type function
- added extensive tests for int/uint types
- added tests for function selectors
- added logic to parse logs that don't depends on a processblock.Log
  instance
- added documentation for the encode_single function
- added function to zero pad to the right

breaking changes:
- `decint` raises if value is a string and out-of-bounds
- `encode_single`:
  - `int` and `uint` raises ValueOutOfBounds instead of EncodingError
  - `string` raises if the supplied value is not a valid utf8
  - `int`, `uint`, `fixed`, and `ufixed` raises if the type `sub` is
  invalid
  - renamed type `real` to `fixed`

Author: hackaugusto